Changing voltage into frequency using OpAmp

Hi, do you have an idea to convert voltage into frequency but using only OpAmps and some basic circuit elements like resistors, capacitors, inductors and diodes?

Or, how can i change the duty cycle of a square wave, with respect to the voltage. In other words, I want a converter like voltage to duty cycle.

I thought to do it like following;

I will take the integral of square wave, and obtain a triangular wave, using integrator circuit.
Then using a comparator circuit, I will reconstruct the square wave.
But, the problem is that, I want 0 duty cycle when I have a 0 voltage to compare, and increasing the voltage, I want to obtain greater duty cycles. But this method gives 50 duty cycle, at voltage 0.

Because it is an high school project and only that chips over 30 years old are allowed to use.
I have voltages up to 20, I am able to split it, so it doesn't matter in what voltages it works. I can obtain any voltages up to 20.
Change doesn't matter, it can be linear, logarithmic, or however you want. I would prefer linear one anyway.
I want to operate it at low frequencies, like that the greatest period will be 1 second.

I hope I have clarified my point. Maybe now you can find a sloution, whereas I didin't want a solution, I have just asked for an idea, so I don't think that you have to know all these details to find it.

Moreover, I have an idea for you to carry salt across the desert using only bunny rabbits. But I should know the followings;

What kind of a salt you want to carry?
Your rabbits are male or female; pink or white?
Are these rabbits are just walking? Or, can they also run and jump?
Why can't you use spaceships?